1. Field of the Invention
The present invention relates to an electric double layer capacitor, and more particularly to an electric double layer capacitor which employs polarizable electrodes consisting of activated carbon or the like impregnated with liquid electrolyte (electrolytic solution).
2. Description of the Prior Art
An electric double layer capacitor employs an electric double layer formed at the interface of a porous solid such as activated carbon and an electrolytic solution, formed by impregnating the electrolytic solution to the activated carbon which is electrochemically stable to the electrolytic solution and is electrically conductive, and is capable of easily realizing a large capacitance of the order of farad (F). The large capacitance which reaches 10 to 50 F/cm.sup.3 is due to the very small thickness comparable to the diameter of a molecule, of the electric double layer that corresponds to the dielectric layer of the ordinary capacitor, and an extremely large effective surface area of the activated carbon. This large capacity makes it possible to use this kind of capacitor as a substitute for a battery which is the power supply of an IC memory or a microprocessor. Since the polarizable electrode and the electrolytic solution does not give rise to a chemical reaction in charging and discharging, differing from a battery which utilizes an electrochemical phenomenon, the life in the use that is accompanied by a repeated charging and discharging, namely, the life for repetitions charging and discharging is much longer than that of a battery.
However, the equivalent series resistance (ESR) of the electric double layer capacitor is high, despite its large capacitance, so its use is limited such as it is not suited for the removal of high frequency components in a signal processing circuit or for a smoothing circuit for rectifier output. Accordingly, exploitation of new usage that makes advantageous use of its ease in realizing a large capacitance and long life for repetitions charging and discharging has been desired.
Aimed at such new uses, technologies that realize a large capacitance and a low resistance (ESR) are disclosed in Japanese Patent Laid Open H4-288361 (1992) filed by the same assignee as the assignee of the present invention, and in Japanese Patent Laid Open S63-226019 (1988). In the polarizable electrode materials described in these publications, as the means for maintaining the state of contact between activated carbon and an electrolytic solution, use is made of a porous solid material obtained by heating the mixture of powder or fibers of activated carbon and phenol resin in the atmosphere of an inert gas, and binding the activated carbon powder (or fibers) with carbonized phenol resin. A polarizable electrode for the electric double layer capacitor is obtained by impregnating the electrolytic solution into the solid material. Compared with a conventional polarizable electrode, namely, a polarizable electrode which is a pastelike mixture of activated carbon and an electrolytic solution, these polarizable electrodes have large density and low resistivity so that it is possible to further increase the capacitance per unit volume of the electric double layer capacitor and further decrease ESR mentioned above. In particular, the polarizable electrode according to the invention disclosed in Japanese Patent Laid Open H4-288361 has a large effect of increasing the capacitance and decreasing the resistance (ESR), and is mechanically strong. It is shown in the above-mentioned publication that it is possible to prolong the life for repetitious charging and discharging of a lead acid storage battery to about six to seven times that of the conventional value by combining the electric double layer capacitor which is formed by impregnating diluted sulfuric acid to the polarizable electrode, in parallel connection with the lead acid storage battery for vehicles. This example of application shows that the electric double layer capacitor is effective in the reduction of size and prolongation of life of an electric energy supply source in an electromechanical energy transducing mechanism such as actuator and motor.
Although new uses have been found through increase of capacitance and reduction of ESR as described in the above, there are still left unsolved problems in order to ensure its reliability. Namely, when the polarizable electrode is placed for a long time in a high temperature environment, the capacitance of the capacitor is decreased and the ESR is increased by the loss of the electrolytic solution. In contrast, in the newly explored use, that is, in the use for composing a power supply for driving the starter motor of vehicles by the combination with the lead acid storage battery, the capacitor is installed in an environment where vibration and impact are prevailing such as the engine room, so that it is indispensable to ensure the reliability under such an environment.
Generally, the electric double layer capacitor has a basic construction in which a pair of polarizable electrodes each impregnated with an electrolytic solution are housed stacked within the inner space of a gasket consisting of a thin frame of insulating material, with a sheet of a separator made of an insulating material placed in between. On the end faces of the gasket, there are respectively provided flat layers of collectors consisting of a conductive material so as to constitute a cover plate and a bottom plate of the gasket. These collectors act as terminal plates to the outside, of the pair of polarizable electrodes, and constitute also the sealing members together with the gasket.
In order to ensure the resistance to vibration and the resistance to impact of the capacitor, external mechanical pressures are applied to the pair of collectors in housing the aggregate in the gasket. For subjecting the collectors to the pressurizing process, the afore-mentioned polarizable electrode based on pastelike mixture according to the conventional technology is more suited than the solid material based polarizable electrode disclosed in the above-mentioned publication. In the latter case it is difficult to realize a sufficient resistance to vibration even after going through the pressurizing process. Namely, since it is necessary to press mold the polarizable electrode in advance, a gap, though slight, needs be provided between the gasket inner surface and the electrodes by considering the accuracy of the machine tool. Accordingly, the pair of polarized electrodes tend to slip out of place in planes parallel to the separator even after they are housed in the gasket, and the resistance to lateral impacts is weak.